Almost everyone could ultimately slay giant (8)
I believe the answer is:
almighty
'giant' is the definition.
(as in an almighty roar)
'almost everyone could ultimately slay' is the wordplay.
'almost' means to remove the last letter (almost the whole word is kept).
'everyone' becomes 'all' (all people).
'could' becomes 'might' (synonyms).
'ultimately' says to take the final letters.
The final letter of 'slay' is 'y'.
'all' with its final letter removed is 'al'.
'al'+'might'+'y'='ALMIGHTY'
(Other definitions for almighty that I've seen before include "All-powerful - like God" , "Supremely powerful" , "Excessive" , "Omnipotent; enormous" , "Huge" .)